Since the end of the 19th century, laminated soft magnetic materials have been used for the construction of single or polyphase transformers and inductors for applications in the usual commercial range of AC supply frequency (from 50 Hz to 1000 Hz) for a wide power range (from 1 VA to several kVA). These isolated laminations present interesting magnetic properties with a high level of induction of saturation (near 1.8 T). The isolation of the laminations also allows the minimization of the magnetic losses because the magnetic flux is circulating in the plane of the laminations (the flux is circulating in two dimensions only). The shapes of the magnetic core are then imposed by this constraint and limited to a toroïd shape, and E, C or I-shape (E-core, C-core or I-core) and all combinations of these topologies.
The cost of the assembly of these devices is relatively high, because the production process needs an important number of steps including lamination forming, punching, mounting and stacking, insertion of the winding and isolation, mounting of the external support and the terminal plate. These transformers are commercially available in standard sizes to cover a wide power range.
One drawback of the lamination use is the generation of an important audible noise for the usual values of frequency of the AC supply systems in the range from 50 Hz to 1000 Hz (50, 60 or 400 Hz for example) see U.S. Pat. No. 529,051 to Inokuti; Yukio et al. “Method of producing low iron loss grain oriented steel having low noise and superior shape characteristics”. The electrical insulation between laminations also reduces in great proportions, the heat transfer between the laminations, and the main part of the heat is circulating in the plane of the laminations, i.e. in two dimensions only. The contribution of the magnetic core for the transfer of the heat generated by the copper losses in the windings and the magnetic losses in the core to the ambiance is therefore limited. In such structures using laminations, the temperature rise between the windings and the laminations remains an important limitation in terms of power to weight ratio.
The variations of the permeability of the magnetic materials used in laminations are very important when saturation is occurring. It is then necessary to oversize the transformers and inductors to avoid saturation in the case of voltage variations of the AC supply. When saturation occurs, the magnetizing current can increase in great proportions and produce an excessive heating of the windings.
The conventional shapes of magnetic cores like E, C and I-configuration cores do not maximize the power to volume and power to weight ratios of the transformers and inductors. In these structures, there are also important magnetic stray fields and leakage flux which circulate in the external environment of the device and can induce parasitic perturbations in electrical or electronic circuits, for example. In applications where the stray magnetic radiation of the transformer or the inductor must be eliminated, magnetic cores with a toroïdal shape are generally used (transformers used in power supplies of audio amplifiers for example) see U.S. Pat. No. 3,668,589 by Wilkinson “Low frequency magnetic core inductor structure”. But the winding process on such a core is difficult and the transfer of the heat generated by copper losses in the windings and magnetic losses in the core to the ambiance, in such transformers and inductors, is not efficient.
The magnetic cores which present a cylindrical symmetry around one main revolution axis with windings enclosed are the best suitable for the realization of transformers and inductors. In such structures, there is an optimal use of the copper volume and a good magnetic coupling between the windings. The power to weight ratio and the power to volume ratio are maximized. But it is impossible to realize this shape of magnetic core with laminations, because in the cores which present a cylindrical symmetry around one main revolution axis with windings enclosed, the magnetic flux is circulating in the three dimensions. It is necessary to use an isotropic soft magnetic material with a low electrical conductivity.
Since 30 years, magnetic cores which present a cylindrical symmetry (Pot-cores for example) have been realized with isotropic sintered soft magnetic materials with low electrical conductivity like ferrites for high frequency power supplies (20 kHz to 300 kHz) see U.S. Pat. No. 4,602,957 to Pollock et al, “Magnetic powder compacts”. The magnetic and thermal properties of these materials are isotropic and their magnetic losses are minimized on a wide range of frequency up to 500 kHz and several Mhz see U.S. Pat. No. 4,507,640 to Rich III et al, “High frequency transformer”. Several distributors, such as Philips, Siemens, etc, are already offering a wide range of standard size ferrite cores with different shapes C, E and I-cores, toroïd cores, ETD-cores and Pot-cores, to realize high frequency transformers and inductors. But, at low frequency, the power to weight ratio of the transformers and inductors is also proportional to the value of the induction of saturation of the soft magnetic material. The induction of saturation of the ferrite material which is relatively low, near 0.4 T, is limiting the use of such a material for applications at low values of frequency used in the conventional AC supplies systems, from 50 Hz to 1000 Hz, for example 50 Hz, 60 Hz and 400 Hz. The use of ferrite materials is then limited to high frequency applications. Because they are sintered, the ferrite materials are also brittle and the size and shape of the cores which can be realized are therefore limited. For example, because these materials are brittle, it is not possible to press cooling fins directly on the cores during forming.
Other kinds of magnetic materials have been proposed for the realization of Pot-Core transformers for low or high frequency applications as disclosed in U.S. Pat. Nos. 4,601,765 to Soileau et al and 4,201,837 to Lupinski. Generally the sintered materials present a high cost of production and the cores which are proposed don't have cooling fins on their external surface to maximize the power to weight ratio.
Several new soft magnetic composites have been recently developed in the domain of powder metallurgy. (ATOMET EM-1 of Quebec Metal Powders Inc for example, see I C. Gélinas, L. P. Lefebvre, s. Pelletier, P. Viarouge, Effect of Temperature on Properties of Iron-Resin Composites for Automative Applications, SAE Technical Paper (7p.) 970421 Eng. Soc. for Advancing Mobility Land Sea Air and Space. Int. Congress Detroit Mich. Feb. 24-27, 1997. In such soft magnetic isotropic materials, the iron flakes are isolated from each other by a resin coating. These materials need a pressing process and a thermal treatment at low temperature. Their cost of production is then reduced. These materials are more adapted to applications where a mass production is necessary, despite the fact that their production cost per kilogram remains higher than the one of laminations (near two times higher).
By using a molding technique, it is possible to realize a core of complex shape in a single operation. It is also possible to machine the soft magnetic composites with conventional tools, while the sintered materials like soft magnetic ferrite can be only rectified with diamond grinding wheels.
The use of the soft magnetic composites for applications in the low frequency domain from 50 Hz to 1000 Hz is not still developed because these materials present a relatively low value of permeability when compared to the value of the permeability of laminations (the relative permeability of the soft magnetic composites is near 200 and 1500 for the conventional grades of laminations).
The magnetic losses at 50 Hz and 60 Hz in the soft magnetic composites are higher than in the soft magnetic laminated materials. (near 5 to 15 W/kg at 1.2 T instead of 2 W/kg for the soft magnetic laminated materials). But at 400 Hz, the magnetic losses of some soft magnetic composites can be 2 times lower see the above-referred technical paper.